- any grass-green, chiefly freshwater algae of the phylum Chlorophyta, often growing on wet rocks, damp wood, or on the surface of stagnant water.
- the algae of the phylum Chlorophyta, which possess the green pigment chlorophyll. The group includes sea lettuce and spirogyra
- Any of various photosynthetic protists belonging to the phylum Chlorophyta. The green algae share many characteristics with plants, notably in their having chlorophylls a and b, in their storage of food as starch, and in the composition of their cell walls from cellulose or other polysaccharides. The green algae show a great variety of body types, ranging from unicellular forms to filaments to leaflike thalli, and many species live in colonies. Green algae also show a variety of reproductive processes, both sexual, by the formation of conjugating gametes or the exchange of nuclei through conjugation tubes, and asexual, by means of spores. Green algae are mostly aquatic, in both freshwater and marine environments. However, many species live on land or in the soil, and even in extreme environments, such as the surface of snow. Green algae are not always green, since they produce carotenoid pigments that can give them orange or red colors. Some lichens consist of a symbiotic relationship between a fungus and a green alga. Sea moss and the common pond scum Spirogyra are green algae. Also called chlorophyte See more at alga.